linux-kernel - Re: [PATCH v2] module: Harden STRICT_MODULE

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Message-ID: <20200811165722.GA7716@willie-the-truck>
Date:   Tue, 11 Aug 2020 17:57:22 +0100
From:   Will Deacon <will@...nel.org>
To:     Jessica Yu <jeyu@...nel.org>
Cc:     Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        peterz@...radead.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>, keescook@...omium.org,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Miroslav Benes <mbenes@...e.cz>,
        Mark Rutland <mark.rutland@....com>,
        Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH v2] module: Harden STRICT_MODULE_RWX

On Tue, Aug 11, 2020 at 06:01:35PM +0200, Jessica Yu wrote:
> +++ Mauro Carvalho Chehab [11/08/20 17:27 +0200]:
> > Em Tue, 11 Aug 2020 16:55:24 +0200
> > peterz@...radead.org escreveu:
> > 
> > > On Tue, Aug 11, 2020 at 04:34:27PM +0200, Mauro Carvalho Chehab wrote:
> > > >   [33] .plt              PROGBITS         0000000000000340  00035c80
> > > >        0000000000000001  0000000000000000 WAX       0     0     1
> > > >   [34] .init.plt         NOBITS           0000000000000341  00035c81
> > > >        0000000000000001  0000000000000000  WA       0     0     1
> > > >   [35] .text.ftrace[...] PROGBITS         0000000000000342  00035c81
> > > >        0000000000000001  0000000000000000 WAX       0     0     1
> > > 
> > > .plt and .text.ftrace_tramplines are buggered.
> > > 
> > > arch/arm64/kernel/module.lds even marks then as NOLOAD.
> > 
> > Hmm... Shouldn't the code at module_enforce_rwx_sections() or at
> > load_module() ignore such sections instead of just rejecting probing
> > all modules?
> > 
> > I mean, if the existing toolchain is not capable of excluding
> > those sections, either the STRICT_MODULE_RWX hardening should be
> > disabled, if a broken toolchain is detected or some runtime code
> > should handle such sections on a different way.
> 
> Hi Mauro, thanks for providing the readelf output. The sections marked 'WAX'
> are indeed the reason why the module loader is rejecting them.
> 
> Interesting, my cross-compiled modules do not have the executable flag -
> 
>  [38] .plt              NOBITS           0000000000000340  00038fc0
>       0000000000000001  0000000000000000  WA       0     0     1
>  [39] .init.plt         NOBITS           0000000000000341  00038fc0
>       0000000000000001  0000000000000000  WA       0     0     1
>  [40] .text.ftrace_tram NOBITS           0000000000000342  00038fc0
>       0000000000000001  0000000000000000  WA       0     0     1

FWIW, I also see the same output as you for both of the GCC 9 and Clang 11
builds I have kicking around, and there are no WAX sections in sight.

Will